Control apparatus for radiofrequency heating



Nov. 14, 1950 B. A. LARYS Filed Nov. 28. 1947 2 Sheets-Sheet l THICKNESSOFMATER/AL l a "Q Qmmz'nboz Bah umir A. Lar ys 13, W

Nov. 14, 1950 B. A. LARYS CONTROL APPARATUS FOR RADIO-FREQUENCY HEATING2 Sheets-Sheet 2 Filed Nov. 28. 1947 THICKNESS 0F MATERIAL Mow/aw w x. LA m. w m.

WWW vase: yh w i? Patented Nov. 14, 1950 CONTROL APPARATUS FOR- RADIOFREQUENCY HEATING Bohnmir A. Larys, Murray Hill, N. J assignor to TheSinger Manufacturing Company, Elizabeth, N. J., a corporation of NewJersey Application November 28, 1947, Serial No. 788,533

6 Claims.

1 This invention relates to systems for producing radio-frequency fieldsand more especially to means for controlling the strength of said fieldsin accordance with the requirements of the load which may be continuallyvarying as, for example, when material of varying number of plies orthickness is being passed through said field.

The use of radio-frequency fields for the heating and bonding ofprogressively-fed dielectric materials is well known. Heretofore it hasbeen customary to change the rate of feed of the material or the platevoltage of the oscillator to compensate for changes in the thickness ofthe material being heated or bonded. This may be a. manual process, inwhich case the operator must anticipate the change in the material andmake the required change in the feed or the voltage at exactly the righttime. This dependence upon the human factor causes spoilage and reducesproduction. It also may be an automatic process, which requires adifficult-toobtain nicety of adjustment between the detection of thechange in the material and the proper correction of the bondingcondition. In those systems in which relays and/or motors are used, thenecessary time lag produces erratic results. When self-excitedoscillators are employed and the plate voltage is altered to efiect achange in power output, difficulty has been experienced due to shifts inthe frequency.

In an effort to overcome the many diificulties which beset theproduction of a uniformly good bond in dielectric materials withvariation in thickness, the problem has been reviewed from thestandpoint of the basic requirements. It has been discovered that, for agiven kind of material, frequency, electrode area, and rate of feed, thecurrent through the material should be maintained substantially constantregardless of any thickness variation, if a uniform bond is to beproduced.

It is, therefore, a primary object of this invention to provide aradio-frequency generating system which shall supply a current ofsubstantially constant amplitude and frequency to a dielectric load.

A further object of this invention is to provide a control system for aradio-frequency generator which may be adjusted to supply a predetermined and substantially constant current to a variable load.

A still further object of this invention is to provide means to detectthe departure and direction of departure of radio-frequency load currentfrom a pre-established value and means responsive to such departure forrestoring the current instantaneously to its preestablished value.

According to the invention, these objects are attained byemploying asmall eXciter oscillator to establish a stable frequency, a driveramplifier, and a final power amplifier connected to the load. The gridof the driver amplifier is variably biased in response to the loadcurrent and either the amplifier output voltage or a stable referencevoltage, and this bias voltage variation controls the grid swing of thefinal power amplifier and thus the voltage output thereof.

With the above and other objects in view as will hereinafter appear, theinvention comprises the combination and arrangement of part here inafterset forth and illustrated in the accom panying drawings from which theseveral fea tures of the invention and the advantages attained therebywill be readily understood b those skilled in the art.

In the drawings:

Fig. 1 is a schematic wiring diagram of a system embodying theinvention.

Fig. 2 is a graphical representation of the relation of load current tothickness variation for good bonding and for a system with noregulation. V

Fig. 3 is a graphical representation of the relation of rectifiedvoltages to thickness variation in the detector portion of the system ofFig. 1.

Fig. 4 is a wiring diagram of a modification of the detector of Fig. 1.

Fig. 5 is a graphical representation of the re lation of the voltages tothickness variation fo the detector of Fig. 4.

Referring to Fig. 2 of the drawin s, a curve A represents the currentvariation with thickness of the material being bonded for ideal con:ditions for good bonding according to this invention. It is seen thatthe current is substantially constant except for very thin materialswhere slightly larger currents are required. This is the result of thegreater cooling effect of the electrodes on the thinner material.

Curve 13 represents the manner in which the current normally falls offas the load thickness increases in the case of a conventionalselfexcited oscillator without any automatic regulation of any kind. Itis readily seen that, without regulation, the current falls off sorapidly that with conditions set for a given thickness, even a slightlythicker material will not be properly bonded.

Thus, to attain good bonding conditions over a range of thickness, 9.control system must be devised to automatically provide for thediscrepancies between Curves A and B. Fig. 1 shows such a system. Thissystem is based upon the proper use of two well-known amplifier-loadeffects, viz. (1) the tendency for load current to decrease as the loadthickness or resistance increases and (2) the tendency for the outputvoltage of the amplifier to increase as the load thickness or resistanceincreases.

Referring now to Fig. 1, a coaxial transmission line I is connected atits load end with a shunt inductance coil 2, a series inductance coil 3,a series variable tuning condenser 4 and electrodes 5 and 6 betweenwhich is disposed the material 7 comprising plies to be bonded. Elements2, 3 and 4 constitute the load-end tuning device which is substantiallythe same as that shown and described in the co-pending U. S. applicationof Joseph P. Graham and Robert D. Lowry, Serial No. 576,657, filedFebruary 7, 1945, now Patent Number 2,473,143, dated June 14, 1949. Theelectrodes 5 and 6 may be either of the reciprocatory or the rollertype.

A pick-up coil 8 is placed in inductive relation to coil 2 and isconnected in series with a diode rectifier 9 and a potentiometer Ii].Similarly, a pick-up coil II is placed in inductive relation to coil 3and is connected in series with a diode rectifier l2 and a potentiometerl3. The potentiometers I0 and [3 are shunted by condensers 0 and I3",respectively, to provide low impedance paths for the high frequencycomponents of the rectified currents. Sliders l4 and I5 are connected bylead I6, and leads I7 and I8 are connected to the ends of the respectivepotentiometers l0 and I3. It will be seen that the polarity of thediodes is such that the voltage across leads IT and I8 is the differencebetween the voltages appearing between each slider and the bottom end ofits respeotive potentiometer. The elements just described define adetector Hi and the single output control voltage therefrom appearingbetween leads H and I8 varies between a negative Value,

through zero, to a positive value as the thickness L of the material 1increases as will now be explained.

In Fig, 3, curve ill represents the variation in the voltage across thepotentiometer 10 as the thickness varies. thickness because it is ameasure of the radiofrequency voltage as the load resistanceincreasestowards open circuit. Curve l3 represents the variation in thevoltage across the potentiometer 3 as the material thickness orresistance increases. It decreases with increase in thickness because itis a measure of the load current which falls off due to increasedthickness. The voltage between leads I! and H3 is representated by thedifference between the ordinates to the two curves Ill and l3 Thus atpoint 20, a thickness is found for which the output voltage is zero. Inthe region N, the output voltage is negative and decreases withincreasing thickness as shown by the vertical lines. In the region P,the output Voltage is positiveand increases with increasing thickness asshown. It will be seen that the thickness value at which the outputvoltage is Zero may be changed by moving either one or both of thesliders l4 and I5. It is also possible to gang the sliders l4 and Hi toa single control for convenience. Ordinarily these are adjusted toproduce zero output voltage for the average thickness to be encounteredin order to minimize the necessary regulation.

It increases with increase in A plate return for these tubes.

Having thus obtained a D. C. control voltage which is a function of theload variations, let us see how this voltage is utilized to produce asubstantially constant current to the load. Referring again to Fig. 1, atriode tube 2| is con-. nected as a conventional tuned-grid, tuned-plateoscillator, and serves as the exciter for the entire system. Thisoscillator 2| may be turned on and off by connecting the cathode toground through the agency of a keying relay 2| A plate coil 22 of saidoscillator is inductively coupled to a grid coil 23 which is connectedto provide push-pull input to grids 24 and 25 of a dual pentode tube 26,connected as a push-pull driver amplifier. A plate coil 27 of saidamplifier is coupled inductively to a grid coil 28 which connects to thegrids 29 and 30 of two triode tubes 3| and 32 connected as a push-pullpower amplifier. The plate coil 33 of said power amplifier isinductively coupled to a coil 34 which i connected to the sending end ofthe coaxial line I referred to hereinbefore.

Thus, the exciter oscillator 2| sets the frequency and the small voltagegenerated thereby is amplified first by the driver amplifier 26 and thenby the power amplifier 3| and 32 until considerable power is availablefor transmission to the load by the line I. Terminals 35, 36 and 3'! areprovided for the application of the proper positive plate voltages forthe exciter, driver and power amplifier respectively; the ground beingthe common negative voltage terminal 66. Terminal 38 is for theapplication of the proper negative grid bias voltage for the poweramplifier.

r These voltages are maintained substantially constant by the use ofconventional regulated power and bias supplies, not shown. Filamentsupply voltages are applied where shown by arrows on the heater andfilament leads. An adjustable screen-grid voltage supply for the driveramplifier 26 is obtained from a potentiometer 6| placed across the platevoltage supply (terminal 36 to ground). A filament transformer 62 haleads 63 and 54 connecting to the filaments of the power amplifier tubes3| and 32. The center-tap B5 of the secondary is grounded to completethe The exciter oscillator 2| is self-biased and the driver amplifier 26obtains its bias in a special manner now to be described.

As described above, leads I? and I8 supply a D. C. voltage whichincreases in a positive direction as the material thickness increases.This voltage is impressed between grid 39 of triode tube 43 and theslider 4| of potentiometer 42. A

bridge-type rectifier 43 receives alternating cur-.

rent voltage from the filament supply 44 and supplie D. C. voltage tothe potentiometer 42 a portion of which voltage is applied as grid biasbetween the slider 4| and the cathod 45. The cathode 45 is grounded anda cathode resistor46 is connected to terminal 47 which is for connectionto the negative side of a conventional plate voltage supply which iswell regulated. Terminal 48 connects to the plate 49 and permitsconnection to the positive side of a conventional regulated platesupply, not shown. A potentiometer 59 is connected across terminals 41and 48 and has its slider 5| connected to the grid 52 of pentode tube 53which is connected as a triode cathode follower. The cathode of tube 53is grounded and a resistor 54 is connected between said cathode andterminal 55 which provides for connection to the negative side of aconventional regulated plate voltage supply, not shown. Terminal 56connects to plate 51 and provides for connection to the positive side ofsaid plate voltage supply. A lead 58 connects from terminal 55 to themid-point 59 of the grid coil 23 of the driver amplifier 26. The dualcathode 60 is grounded and, thus, it will be seen that the outputvoltage of the .cathode follower tube 53 which appears across theresistor 54 is impressed directly as bias voltage on the grid circuit ofthe driver amplifier 26.

Operation It will be apparent from the above description of the circuitthat there is a certain thickness of material 1 for which the D. C.voltage output of the detector is zero. This value of thickness may beshifted by manual control of either or both sliders l4 and I5.

First, let us suppose that conditions are such that no net voltageappears across leads I! and I8. In this case, the setting of the slider4| of potentiometer 42 determines the voltage on the grid 39 whichdetermines the plate current of tube 40 and thus the voltage drop in theresistor 46. The voltage impressed on the grid 52 of the cathodefollower 53 is made up of a positive component taken from thepotentiometer 50 by the slider 5|, and a negative component equal to thevoltage drop in the resistor 46. This voltage will determine the valueof the plate current of the cathode follower tube 53 which, in turn,fixes the value of the voltage drop in the resistor 54. This voltage isthe bias voltage on the grids of the driver amplifier 26 and determinesthe output voltage thereof. Thus, potentiometer sliders 4| and 5| may beadjusted so that the proper current is being supplied for a giventhickness and kind of material to make a satisfactory bond. If, now,thicker material is encountered a more positive voltage appears as theoutput voltage of the detector I9 and this provides a less negative gridvoltage for the tube 40. The plate current of tube 40 increases and thisincreases the negative voltage applied to the grid 52 of the cathodefollower. This 1owers the negative grid bias voltage supplied to thegrids of the driver amplifier b the cathode follower and provides ahigher output voltage from said amplifier. This higher voltage increasesthe voltage drive on the power amplifier and the output voltage isboosted so that the current increases substantially to its originalvalue. If the material gets thinner, the reverse action takes placebecause of the ability of the detector to discriminate between thedirections of thickness change. In general, the action is such that thetendency for the load current to change is substantially counteracted bythe voltage change produced by the grid bias control as described. Thatis to say, there tends to be a constant voltage gradient maintained inthe material by automatic voltage control responsive to thicknesschange.

It is also possible to obtain regulation by using only one detector coill I and comparing the rectified voltage obtained therefrom with aconstant D. C. voltage obtained from a battery or similar source ofconstant voltage. Accordingly, Fig. 4 shows a detector, 19 modified fromthe one shown in Fig, 1 by the substitution of a battery 61 for thepick-up coil 8 and rectifier 9.

It will be seen that the operation is the same as above explained inconnection with the detector I9 except that instead of the risingvoltage characteristic (III of Fig. 3) of the rectified voltage suppliedby the coil 8, a constant voltage characteristic supplied by the battery61 is used. This is shown in Fig. 5 as curve 61*. The regions ofnegative and positive control voltages are shown at N and P, while atpoint 20 the thickness corresponds to zero control voltage, or noregulation. A control voltage increasing positively with increasingthickness is obtained as shown by the vertical lines of Fig. 5, andoperation of the circuit is the same as explained above.

It is to be noted that no motors or relays are used and the speed ofresponse is, therefore, limited only by the time taken for currents totraverse the circuits, which time, relative to the rate of feed of thematerial, is practically instantaneous.

By employing an exciter which need produce only a very small outputpower and by employing amplifiers between said exciter and the load itis possible to make the frequency very stable and practicallyindependent of the variations in the load. This insures that thefrequenc will remain within the band allocated for the equipment andthat the heating effectiveness will not be changed by change offrequency. Inasmuch as it works into a high impedance grid circuit, thedetector unit [9 does not supply much current, and may be made verysmall, physically, by the use of miniature components. It may,therefore, be placed very close to the electrodes 5 and 6, in closeproximity with the load itself.

Having thus set forth the nature of the invention, what I claim hereinis:

1. In a control system for maintaining constant the radio-frequencycurrent to a variablethickness dielectric load, a radiofrequencygenerator, a dialectric load, means for deriving a first voltageproportional to the current to the load, and a second voltageproportional to the output voltage of said generator, means forrectifying said voltages, means for combining preselected portions ofsaid rectified voltages to produce a single voltage which is a functionof the thickness of the load, and means responsive to said singlevoltage for maintaining substantially constant the current to the load.

2. In a bonding apparatus, an oscillator, an amplifier fed from saidoscillator, a load comprising plies of material of varying thickness, atransmission line for transferring current from said amplifier to saidload, means for deriving a first D. C. voltage proportional to thecurrent to the load, means for deriving a second D. C. voltageproportional to the voltage at the load-end of said transmission line,means for combining these D. C. voltages to produce a single voltage,and means for impressing said single voltage on the grid of saidamplifier.

3. In a system for the radio-frequency heating of dielectric material,an oscillator, a driver amplifier fed from said oscillator, a poweramplifier fed from said driver amplifier, a variable load including saiddielectric material, fed from said power amplifier, means for deriving afirst D. C. voltage proportional to the radio frequency current to theload, means for deriving a second D. C. voltage proportional to theradio frequency voltage output of said power amplifier, means forcombining said voltages to produce a single voltage, means foramplifying said single voltage, and cathode follower means fortransferring the amplified voltage to.the grid circuit of said driveramplifier whereby to vary the input voltage to the power amplifier and 7thereby the output voltageto the load to maintain the load currentsubstantially constant.

4. In a radio-frequency system, a radio frequency' generator including agrid-modulated amplifier, a variable load connected to receive currentfrom said generator, means for deriving a D. C. voltage proportional tothe radio-frequency current to the load, means for deriving a D. C.voltage proportional to the radio-frequency voltage output of saidgenerator, means for combining said D. C. voltages to produce a singlevoltage, and means for amplifying said single voltageand cathodefollower means for transferring said amplified voltage to the gridcircuit of said grid-modulated amplifier.

5; In,a radio-frequency apparatus for heating dielectric material, aradio-frequency voltage generator, a valuable load including dielectricmaterial, means for transferring current from said generator to saiddielectric material, means for deriving a first D. C. voltageproportional to said current, means for deriving a second D. C. voltageproportional to the output voltage of said generator, means forsubtracting said D. C. voltages to produce a single control voltage, andmeans responsive to said single control voltage for varying the outputvoltage of said generator to maintain substantially constant the currentto said dielectric material.

6. In a sysem for controlling the transfer of radio-frequency energyfrom a generator to a load, the resistance of which is subject to randomvariation, means for pickingup a voltage proportional to the outputvoltage of said generator, a load, means for picking up a voltageproportional to the current to said lead, separate rectifying means forrectifying said voltages, means for selecting a portion of each of saidrectified voltages, and means for subtracting said portion voltages toproduce a single control voltage which is a function of said randomvariation of the load resistance.

' BOHUMIR A. LARYS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,131,366 Black Sept. 27,19382,205,424 Leonard June 25, 1940 2,245,353 Morlock June 10, 19412,251,277 Hart, Jr. et a1. Aug. 5, 1941 2,282,377 Place May 12, 19422,331,360 Tuckerman Oct. 12, 1943 2,416,172 Gregory et a1 Feb. 18, 19472,424,905 Scheldorf July 29, 1947

